Just picked up a 05 with a Fabtech lift on it. Im getting a bad vibration when I put it in 4x4 from about 0 to 30 mph. The 1st time I tried out the 4x4 it smelt like burning rubber. 4x4 works great except for the vibrations. In 2 wheel drive it does fine. This is my 1st IFS truck, Im not sure where to start.

 

Are you engaging 4wd on dry pavement? If yes, then expect some degree of driveline bind. Most 4wd systems, the F150 included, require a small amount of slip especially when turning to keep the driveline components happy.

However if you're getting this vibration while in snow, mud, etc then you likely have half-shaft or differential issues.

 

How would I diagnose a bad half shaft, I checked my tcase oil and it looks like it needs changed. I checked the front diff also, and the oil look creamy. Can I rule out the front drive shaft since the vibration is gone in 2wheel.

 

You didn't answer the pavement question.
Get the truck on jack stands or a lift and start looking for worn parts and play in the joints and shafts.

 

Im using the 4x4 in snow.

 

When I kick it in it feels like im driving over the rumble strips on the road.